Typical human powered bicycles are propelled by pedals mounted on cranks at opposite ends of an axle. The cranks drive the axle, on which usually one or more sprockets are mounted that engage a chain to transfer the rotary motion to a rear wheel. The part of the bicycle frame where the axle is mounted is called the bottom bracket. Here, the axle is rotatably held in a bearing, so that it can rotate with very low friction.
For mounting the axle with low friction, ball bearings are employed, consisting of inner and outer races with interposed bearing balls. In many known bottom bracket assemblies, the inner race is formed as a part of the axle or spindle. However, this has proven to create a number of problems. Bearing races must be very hard, and hence brittle, due to high contact stresses present in rolling element ball bearings. Brittleness, however, is highly undesired for the spindle because it reduces the ability of the spindle to absorb high impact energies occurring in some situations.
One example of a bottom bracket bearing assembly where the inner races of ball bearings are formed by the spindle itself is described in French patent 270,698 to Bazille.
Other known bottom bracket bearings of the prior art make use of sealed cartridge bearings, which integrate the two races with the bearing and are available as readily assembled units. Cartridge bearings can easily be replaced when worn or damaged. The successful use of cartridge bearings in bicycle bottom brackets, however, requires the designer to ensure that the inner and outer bearing races are correctly aligned. If axial loads lead to misalignment of the bearing races during assembly or use of the bottom bracket, damage is likely to occur.
If the both the inner and outer bearing races of a cartridge bearing are fixed axially in the bottom bracket assembly, correct alignment can only be ensured by very tight control of manufacturing dimensions and tolerances. The distance between the fixed positions of the two outer bearing races must be nearly identical to the distance between the inner bearing races. This has proven to raise manufacturing cost and make the overall bottom bracket design intolerant to manufacturing and assembly variations.
A number of known bottom bracket assemblies of the prior art solve the problem of correct alignment of the inner and outer bearing races by allowing the inner races to float on the spindle while the outer races are fixed in an outer bracket portion, typically being clamped between a shell and adapters inserted at both ends. However, this design makes it difficult to efficiently incorporate crank stops into the bottom bracket. Crank stops are a desirable addition to a bottom bracket because they ensure correct and consistent axial alignment of the installed crank arms.
U.S. Pat. No. 3,903,754 to Morroni and U.S. Pat. No. 4,093,325 to Troccaz both show bottom bracket assemblies where a spindle is rotatably mounted in the bottom bracket by cartridge bearings. The inner races of the cartridge bearings are spaced apart by shoulders on the spindle while the outer races are spaced apart by a tubular spacer or shell. Axial alignment of the inner and outer bearing races in this case can only be accomplished by matching the width of the shoulders and the tubular spacer, using very tight dimensional tolerances.
U.S. Pat. No. 4,545,691 to Kastan shows a bottom bracket assembly for a bicycle where the inner races of the cartridge bearings are fit on the axle and are clamped between a shell and an adapter to fix their axial position. The outer races are butted against shoulders located in the bearing adapters. During initial assembly, the bearing adapters are allowed to float. The adapters are then tightened into axial alignment with the inner bearing races by a tapered split ring.
The present invention aims to provide a bottom bracket assembly for a bicycle, where in a simple construction cartridge bearings can be used advantageously.